What are the differences between interstitial fluid (ISF) and blood glucose (BG) readings?

Glucose levels can be measured from the bloodstream, or from the interstitial fluid (ISF), which surrounds the body’s cells. There is a delay in ISF glucose response to changes in blood glucose,1 which is unlikely to impact routine day-to-day treatment decisions. The time lag  2.4 ± 4.6 minutes for adults and 2.1 ± 5.0 minutes for pediatrics.2 ISF glucose and BG measurements taken simultaneously won’t always match, and, in fact, are likely to be different3


1. Rebrin K, Sheppard NF Jr, Steil GM. Use of subcutaneous interstitial fluid glucose to estimate blood glucose: revisiting delay and Sensor offset. J Diabetes Sci Technol. 2010;4(5):1087-1098. 

2. Alva S, Bailey T, Brazg R, et al. Accuracy of a 14-Day Factory-Calibrated Continuous Glucose Monitoring System With Advanced Algorithm in Pediatric and Adult Population With Diabetes. J Diabetes Sci Technol. 2022;16(1):70-77.

3. FreeStyle Libre flash glucose monitoring system User's Manual.

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